A flashlight is 40.0 cm from a double convex lens. The image of the flashlight is observed at a position 30.0 cm on the other side of the lens. What is the focal length
of the lens?
1) 0.0580cm
2) 10.0cm
3) 13.3cm
4) 17.2cm

4.) 17.2cm

Explanation:

A flashlight is 40.0 cm from a double convex lens. The image of the flashlight is observed at a position 30.0 cm on the other side of the lens. What is the focal length?

Given the following :

Object distance(u) = 40cm

image distance (v) = 30cm

Focal length(f) =?

Using the mirror formular:

f = uv / (v+u)

f = (40 * 30) / (40 + 30)

f = 1200 / 70

f = 17.142857 cm

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